Oxidative coupling of methane—comparisons of MnTiO3–Na2WO4 and MnOx–TiO2–Na2WO4 catalysts on different silica supports

The oxidative coupling of methane (OCM) converts CH4 to value-added chemicals (C2+), such as olefins and paraffin. For a series of MnTiO3-Na2WO4 (MnTiO3-NW) and MnOx-TiO2-Na2WO4 (Mn-Ti-NW), the effect of loading of MnTiO3 or MnOx-TiO2, respectively, on two different supports (sol–gel SiO2 (SG) and commercial fumed SiO2 (CS)) was examined. The catalyst with the highest C2+ yield (21.6% with 60.8% C2+ selectivity and 35.6% CH4 conversion) was 10 wt% MnTiO3-NW/SG with an olefins/paraffin ratio of 2.2. The catalyst surfaces with low oxygen-binding energies were associated with high CH4 conversion. Stability tests conducted for over 24 h revealed that SG-supported catalysts were more durable than those on CS because the active phase (especially Na2WO4) was more stable in SG than in CS. With the use of SG, the activity of MnTiO3-NW was not substantially different from that of Mn-Ti-NW, especially at high metal loading.

www.nature.com/scientificreports/ with a minimum C 2 H 4 yield of 25% required to make the method economically feasible (35-50% would be more practical) 6 . Generally, to be commercially attractive, C 2+ selectivity and CH 4 conversion in OCM must exceed 80% and 30%, respectively 7 , which has stimulated research for many years 8 . However, despite exhaustive efforts, the OCM reaction still lacks a highly active and stable catalyst with high performance for C 2+ formation. Previous studies have indicated that Na 2 WO 4 -MnO x -SiO 2 -based catalysts were among the most active ones that yielded high C 2+ selectivity at high CH 4 conversions, resulting in a C 2+ yield of 10-35% [9][10][11][12][13][14][15][16][17][18][19] . The high performance of these catalysts resulted from a complex combination of several factors, as inferred from literature. Some interesting facts on the active components are detailed as follows: i. Na atoms are necessary for the phase transition of amorphous SiO 2 to crystalline α-cristobalite at low calcination temperature (800 °C) 20,21 . It is noteworthy that the usual temperature for this phase change is approximately 1500 °C 22 . Additionally, the mobility of Na + species during OCM at high temperatures (750-850 °C) can create active sites on the catalyst surface, facilitating hydrogen abstraction of CH 4 23 . Moreover, it was suggested that Na + aids in the stability of the active WO 4 phase 9,24 . Furthermore, it was claimed that active Na 2 O species can be generated in small amounts in the catalytically relevant temperature regime (above 600 °C), and these Na 2 O species are responsible for high activity and C 2+ selectivity 25 . ii. In a study of Na 2 WO 4 /SiO 2 using temporal analysis of products (TAP) and steady-state OCM reaction studies, the Na-WO x sites on the surface were indicated to be responsible for the selective activation of CH 4 to C 2 H x and over-oxidization of CH y to CO. On the other hand, the molten Na 2 WO 4 phase promotes the oxidative dehydrogenation of C 2 H 6 to C 2 H 4 , but it is also responsible for the over-oxidation of CH 4 to CO 2 26 . iii. WO 4 and MnO x are the crucial active components for the generation of C 2+ products. They cooperate during the reaction as follows. Initially, the O 2species associated with the surface WO 4 (W 6+ ) sites, especially the tetrahedral form 27 , activates CH 4 into a gaseous methyl radical. As a result, the W 6+ center is transformed to W 5+ with one chemical bond of W-OH. Thereafter, the oxidation of W 5+ to W 6+ occurs through electron transfer from W 5+ to Mn 3+ , subsequently reducing Mn 3+ to Mn 2+ . Finally, the oxidation of Mn 2+ to Mn 3+ occurs via reaction with gaseous oxygen, which generates an OH radical and an active oxygen atom [28][29][30] . In other words, the MnO x species boosts the oxygen mobility in the catalyst, resulting in an improved exchange of gaseous and surface oxygen 31 . Additionally, an in-depth study of Mn addition to 5Na 2 WO 4 /SiO 2 catalyst revealed that Mn enhances the formation of both dissolved O 2 and lattice atomic O species, which are responsible for catalyzing the OCM reaction. Therefore, the CH 4 conversion toward C 2+ formation is improved 32,33 . iv. The addition of TiO 2 into the MnO x -Na 2 WO 4 /SiO 2 catalyst further reduces the temperature of CH 4 activation in the OCM reaction to approximately 650-720 °C. This is probably due to the formation of a MnTiO 3 phase that enables the transition of Mn 2+ to Mn 3+ at a lower temperature than that of the MnO x phase 34 . v. The α-cristobalite SiO 2 is considered to assist CH 4 activation (although the underlying mechanism is unclear) 15,20 , but upon calcination (starting with amorphous SiO 2 ), the surface area of the support declines substantially 14,35 . Accordingly, some reports have shown that the phase transformation during the reaction causes catalyst deactivation 27,36 . Other causes for catalyst deactivation may include the decomposition of active phases (such as Na 2 WO 4 ) 37-39 and/or loss of Na 2 O due to sublimation during the reaction 25 .
In several other studies, catalysts containing Mn, Na, Ti, W, and/or mesoporous SiO 2 have been claimed as highly active for OCM 27,29,34,[38][39][40][41][42] with MnTiO 3 , MnO x , and TiO 2 components being recommended as critical active phases for CH 4 activation. Many studies have attempted to further improve the C 2+ selectivity and CH 4 conversion of Na 2 WO 4 -MnO x -SiO 2 -based catalysts by understanding the mechanism over the surface. For example, a study on the addition of H 2 O to the testing system revealed that H 2 O did not change the overall scheme of product formation but it was able to reduce the contribution of direct oxidation of CH 4 to CO 2 43 . The effect of pressure was studied on the performance of Mn/Na 2 WO 4 /SiO 2 , which revealed that the increase of pressure leads to higher C 2+ selectivity and can accelerate unselective gas-phase reactions more than surface catalyzed processes 19 . It is also possible that the CH 4 conversions of 60-75% range could be obtained at a maximized C 2+ yield in each specific reactor setup 19 . Our previous investigation of MnTiO 3 and Na 2 WO 4 on different silicabased supports (fumed SiO 2 , MCM-41, and SBA-15) showed that the presence of MnTiO 3 and Na 2 WO 4 on SBA-15 substantially increased CH 4 activation of CH 3 and H radicals, causing the OCM reaction to efficiently generate C 2+ hydrocarbons 27 . However, the MnTiO 3 nanocomposite has been less studied than the MnO x + TiO 2 nanocomposite. Furthermore, the preparation of sol-gel SiO 2 has never been combined with such catalysts. We have speculated that a catalyst prepared using the sol-gel SiO 2 may resist catalyst deactivation better when fumed SiO 2 is used. Therefore, the current study aimed for a systematic comparison of the activity/selectivity of MnTiO 3 -Na 2 WO 4 and MnO x -TiO 2 -Na 2 WO 4 supported on different silica-based supports (sol-gel SiO 2 and commercial fumed SiO 2 ). The results should reveal: (i) Whether the MnTiO 3 phase indeed functions better in the OCM reaction than the nanocomposite mixture of MnO x and TiO 2; and (ii) Whether the type of SiO 2 support has a substantial influence on catalytic performance.

Results and discussion
Activity of catalysts in the OCM reaction. The nanocomposite MnTiO 3 -NW/CS, MnTiO 3 -NW/SG, and Mn-Ti-NW/SG catalysts, containing 0-20 wt% of MnTiO 3 or Mn-Ti, respectively, were tested for the OCM reaction (Table 1, Supplementary Figure S1). Considering the catalyst performance in terms of C 2+ yield per catalyst mass, when the amount of MnTiO 3 or Mn-Ti increased from 0 to 5 or 10 wt%, respectively, the corresponding activities suddenly increased, but then maintained the maximum values for higher loadings. The high- For the OCM reaction, designing a catalyst with high olefin selectivity is still challenging. Olefins, especially ethylene and propylene, have been important raw chemicals since the beginning of the chemical industry in the 1920s. Various downstream products of olefins, such as polyethylene, polypropylene, polyvinyl chloride, and ethanol), are utilized worldwide 44 . In Table 1, the catalytic performance is presented in terms of olefins/paraffin selectivity and olefins/paraffin ratio. It is important to note that the number (5, 10, 15, and 20) preceding each catalyst name refers to the loading of MnTiO 3 or Mn-Ti, with "0wt%" omitted. The results indicated that NW/ CS and NW/SG had relatively low olefins/paraffin ratios of 1.3 and 1.2, respectively. The 5MnTiO 3 -NW/CS and 10MnTiO 3 -NW/CS catalysts had relatively high olefins/paraffin ratios of 2.1, and 2.2, respectively, with the highest C 2+ yield of 20.6%. When the MnTiO 3 amount increased over 10 wt%, the olefins/paraffin ratio slightly decreased, corresponding to a higher CO x selectivity. For MnTiO 3 -NW/SG and Mn-Ti-NW/SG, the olefins/ paraffin ratios changed negligibly (2.2-2.3) as the amount of MnTiO 3 and Mn-Ti increased. The difference in the behaviors of olefins/paraffin ratio at high loadings could be related to the distribution of the active metals on each support. The active metals of the catalysts using SG could be more well-dispersed due to the high porosity of the SG (see more detail in the discussion of Scheme 1). Considering the performance between MnTiO 3 -NW and Mn-Ti-NW on SG, at the same loading, the performance between these two had no significant difference, except at 5wt% loading 5MnTiO 3 -NW/SG exhibited a slightly lower performance. The catalysts producing the maximum yield in each group (5MnTiO 3 -NW/CS, 10MnTiO 3 -NW/SG, and 20Mn-Ti-NW/SG) exhibited high levels of dehydrogenation resulting in olefins/paraffin ratios of 2.1-2.3. However, when considering the C 2+ formation rate (r C2+ )-total moles of C 2+ per total moles of MnTiO 3 or (Mn + Ti) and Na 2 WO 4 per h-of each catalyst (see Table 1), 5MnTiO 3 -NW/CS, 5MnTiO 3 -NW/SG, and 5Mn-Ti-NW/SG exhibited the highest r C2+ in each group. This suggests that the accessible active sites of the catalysts at high loadings (>5 wt%) are limited, possibly because of the low surface area and lack of pores of the catalysts. Nonetheless, this present work considers the catalyst performance in terms of C 2+ yield per catalyst mass, and thus 5MnTiO 3 -NW/CS, 10MnTiO 3 -NW/ SG, and 20Mn-Ti-NW/SG were chosen for further characterization.
Catalyst characterization. The most promising catalysts from Sect. "Activity of catalysts in the OCM reaction" were further analyzed to explain their performance via their properties. The selected catalysts included the basic catalysts (NW/CS and NW/SG), the catalysts producing the maximum yield in each group (5MnTiO 3 -NW/ CS, 10MnTiO 3 -NW/SG, and 20Mn-Ti-NW/SG), and one other catalyst from each group (20MnTiO 3 -NW/CS, 20MnTiO 3 -NW/SG, and 5Mn-Ti-NW/SG).
The XRD patterns of these catalysts are collected in Supplementary Figure S2, with the assignment of XRD peaks to crystalline phases summarized in Supplementary Table S1. All catalysts exhibited XRD peaks Weak signals of the α-tridymite phase in all catalysts and strong signals of quartz were perceived in 10wt% and 20wt%MnTiO 3 -NW/SG. The catalytic tests (Table 1) indicate that the catalysts containing MnTiO 3 , MnO x , TiO 2 , and α-cristobalite had high C 2+ yields; thus, these crystalline phases seem essential in enhancing C 2+ formation. The morphology of the catalysts imaged by FE-SEM is presented in Fig. 1 and the average particle size of each catalyst determined using ImageJ software is presented in Supplementary Figure S3-S12 and Table S2-S11. The CS support appeared spherical with an average particle size of 34 nm (Fig. 1a), whereas SG contained multiple layers (average thinness of 0.4 µm) with porosity ( Fig. 1e). All prepared catalysts had similar coral-like particles. However, the average width of particles of catalysts prepared from CS (approximately 0.3-0.5 µm) was somewhat smaller than those from SG (approximately 0.4-0.6 µm). Upon the addition of different quantities of MnTiO 3 or Mn-Ti to the catalysts, the particle size and shape on the micrometer-scale changed negligibly. The elemental distribution and amount of Mn, Ti, Na, W, and Si in 10MnTiO 3 -NW/SG (Fig. 2) and the other selected catalysts (Supplementary Table S12) were examined by FE-SEM with EDX. Each element was well-dispersed over the surface of the catalysts, which facilitated the activation of CH 4 10 .

Scheme 1.
Models for the stability of the catalysts using CS-and SG-supports. www.nature.com/scientificreports/ The structural morphologies of 10MnTiO 3 -Na 2 WO 4 /SG were characterized using HR-TEM as shown in Fig. 3. As seen in Fig. 3 50 . The HR-TEM results confirm that the active phases distributed throughout the SG support and 10MnTiO 3 -Na 2 WO 4 /SG are nanocomposite due to the nano-scale structures 51 .
The BET surface area, pore-volume, and pore size of all catalysts determined by N 2 -physisorption are presented in Table 2 (isotherms are shown in Supplementary Figure S13). SG had the highest BET surface area (409.2 m 2 g -1 ), followed by CS (89.6 m 2 g -1 ). After using the supports for catalyst preparation, the BET surface areas were largely reduced (0.7-5.7 m 2 g -1 ). The surface area of similar catalysts was also reduced after calcination (2.9-8.6 m 2 g -1 ; amorphous SiO 2 support 89.6 m 2 g -1 ) 10,38 . The tremendous reduction of the surface area of the supports (i.e., SG and CS) generally occurs when preparing a catalyst containing Na 2 WO 4 /SiO 2 . This is because the structure of the support (e.g., porous material and/or nanoparticle) has completely changed from  www.nature.com/scientificreports/ nanoparticles (30-50 nm) to large grain sizes (approximately 0.2-1.0 µm) as indicated by the SEM images in Fig. 1 of an α-cristobalite-containing catalyst without internal pores (i.e., non-porous material) 27,52 . Thus, a very small surface area (e.g., < 6 m 2 /g) is obtained. The catalysts containing MnTiO 3 exhibited smaller surface areas, pore-volumes, and pore sizes than those without MnTiO 3 . Similarly, the addition of Mn + Ti produced catalysts with a small surface area (3.2-3.4 m 2 g -1 ), but still larger than those containing MnTiO 3 . Nevertheless, there was a negligible difference between the pore volumes and pore sizes of catalysts with and without Mn + Ti. Moreover, the increased loading of MnTiO 3 from 10 to 20 wt% on NW/SG or Mn + Ti from 5 to 20 wt% on NW/SG did not significantly change the surface area. Based on the N 2 -physisorption isotherm analysis in Supplementary Figure S13, and according to the IUPAC classification, the SG support is specified as type IV (mesoporous material, containing pore size of 5-10 nm) with H1 hysteresis, while the CS support and all the prepared catalysts showed an indistinct hysteresis loop. However, after careful analysis of the plots shown as inserts of Supplementary Figure S13 and the SEM images ( Fig. 1), the CS support and the catalysts appeared to be non-porous with rough surfaces, which would be classified as type II (nonporous material). The FTIR patterns of selected catalysts producing the maximum yield in each group (5MnTiO 3 -NW/CS, 10MnTiO 3 -NW/SG, and 20Mn-Ti-NW/SG) are displayed in Fig. 4. The FTIR peaks at 970, 880, and 740 cm -1 correspond to Si-O-(H-H 2 O) 28 , Si-OH 53 , and Si-O-C 54,55 bonds, respectively. The FTIR peak at 622 cm -1 for all Table 2. BET surface area, pore volume, and pore size of catalysts.

Catalyst
BET surface area (m 2 g -1 ) Pore volume (cm 3 g -1 ) Pore size (nm) . Previous studies suggested that the amorphous SiO 2 can transform to the α-cristobalite SiO 2 at 800 °C if Na is present during the calcination 20,21 . To confirm that, CS, Na/CS, and NW/CS were prepared using the same procedure as 5MnTiO 3 -NW/CS and analyzed using FTIR as shown in Supplementary Figure S14. The peak at 622 cm -1 that specifies the α-cristobalite SiO 2 phase was also observed when Na was present (i.e., Na/CS and NW/CS). This is in good agreement with the reports 21 . An FTIR peak at 695 cm -1 was only observed for 10MnTiO 3 -NW/SG, indicating the presence of quartz (SiO 2 ) 56 . The FTIR peak at 525 cm −1 is associated with the bending vibration mode of O-Mn-O 57 , indicating the presence of MnO 2 . The intense peak at 590 cm −1 originates from the vibration of the O-Ti-O bond 58 , which was detected in the TiO 2 -containing catalyst. The peak at 1632 cm −1 was assigned to the O-H bending mode, due to moisture 59 .
The surface composition of the catalysts was analyzed by XPS (Supplementary Table S13 and Supplementary Figure S15-S19). The curve-fitted and quantified XPS peaks of Na 1s, W 4f., Si 2p, Ti 2p, and Mn 2p are presented in Supplementary Table S13. The XPS peaks were considered as a mixture of Gaussian and Lorentzian functions (80:20 ratio) 60 . Na is a component of Na 2 WO 4 (Supplementary Figure S15). The binding energies of W 4f 7/2 and W 4f 5 In the OCM mechanism, the oxygen species are crucial for CH 4 activation. The XPS spectra and the O 1 s binding energies of the different catalysts are presented in Fig. 5 and Table 3, respectively. Overall, three types  . Based on the activity results in Fig. 5, catalysts containing M-O (i.e., XPS peak around 530.5-530.9 eV) exhibited high CH 4 conversion. This implied that a catalyst with this specific oxygen species promotes CH 4 activation as the lattice oxygen of the catalyst surface is easily reacted and re-populated (high oxygen mobility).

Stability of catalysts in the OCM reaction.
The stability of 10MnTiO 3 -NW/CS, 5MnTiO 3 -NW/SG, and 20Mn-Ti-NW/SG, catalysts that had produced the highest C 2+ yields, was further tested for over 24 h (Fig. 6). The three catalysts had similar performance with the maximum C 2+ yield (~ 21-23%) obtained within 3-4 h. After that, performance slightly diminished, especially for CH 4 conversion and C 2+ yield. The reduction levels in terms of C 2+ yield for MnTiO 3 -NW/CS (Fig. 6a), MnTiO 3 -NW/SG (Fig. 6b), and Mn-Ti-NW/SG (Fig. 6c) were 15.3%, 8.4%, and 10.5%, respectively. Moreover, 20wt% Mn-Ti-NW on CS (20Mn-Ti-NW/CS) was prepared and tested for reaction to compare its performance with that of 20Mn-Ti-NW/SG (see Supplementary Figure S20). As observed, the activity of Mn-Ti-NW/CS decreased faster than the other catalysts, similar to the previous reports 27, [37][38][39] . This confirms that the MnTiO 3 -NW/SG catalyst was the most active and durable catalyst among all prepared.
The XRD patterns of fresh and used catalysts are presented in Fig. 7 (for peaks assignments see Supplementary  Table S1). Some XRD peaks indicated changes. First, the Na 2 WO 4 peaks in used MnTiO 3 -NW/CS and Mn-Ti-NW/SG catalysts had disappeared, which could be a result of the destruction of crystalline Na 2 WO 4 11,68,69 . This may be related to the reduction in catalytic activity. Second, the quartz phase was found in the used MnTiO 3 -NW/ CS catalyst and the presence of α-tridymite became more pronounced in the used Mn-Ti-NW/SG. Thus, the activity of the catalysts was reduced, because these two phases do not promote methane activation. For the www.nature.com/scientificreports/ MnTiO 3 -NW/SG catalyst, the XRD peaks after reaction remained practically unaffected, confirming the excellent catalytic stability of MnTiO 3 -NW/SG with no activity loss as changes in crystalline phases were absent. The morphologies of fresh and used catalysts once more analyzed by FE-SEM (Fig. 8) and the average particle size of used catalysts were determined using ImageJ software and is presented in Supplementary Figure S21-S23 and Table S14-S16. The metal distribution of each catalyst was also characterized by FE-SEM with EDX (Supplementary Table S12). When the FE-SEM images of fresh and used (24 h) catalysts were compared, the particle shapes appeared very similar. Nevertheless, the particle size of each used catalyst (approximately 1.2-1.6 µm) was more than double of each fresh catalyst (approximately 0.3-0.6 µm). This is paralleled by a reduction in  www.nature.com/scientificreports/ surface area, so that performance in OCM decreased after testing for several hours. Moreover, the increase in particle size may also result in a partial loss of the catalytic interface between the active components for activating methane (i.e., Na 2 WO 4 and Mn 2 O 3 ) 68,69 , thereby causing decreased catalytic activity. The EDX images of the used catalysts (Supplementary Table S12) indicated the presence of Mn, Ti, Na, and W elementals on the catalyst surface. The distribution and concentration of the active elements did not significantly change, in good accordance with the stability tests. Another reason why the catalysts prepared using SG were more stable than those using CS is presented in Scheme 1. According to the textural properties in Table 2, SG has a larger surface area with high porosity, while CS has no internal pores. Thus, during impregnation, the active components were impregnated inside the pores of SG before the transformation of amorphous SiO 2 to α-cristobalite, so that the active components were widely dispersed inside the catalyst's pores. In contrast, the active components impregnated on CS were dispersed only over the surface of CS (like a thin film coating 69 ), since it is non-porous (see CS in Scheme 1). According to some reports, the crystalline phase of Na 2 WO 4 disappeared due to slow evaporation within several hours of the stability test 11,68,69 and/or it transforms to another phase (MnWO 4 ) 68 . Notably, the melting temperature of Na 2 WO 4 is 698 °C, while the reaction temperature for OCM is over 700 °C due to exothermicity 11 . Therefore, it seems easier to lose Na 2 WO 4 from the CS-supported than from the SG-supported catalyst.
The FTIR spectra of fresh and used catalysts are shown in Fig. 9. After the catalysts had been used, the FTIR peak of Si-O-(H-H 2 O) disappeared, suggesting that the H 2 O attached to Si-O-H was removed. Additionally, there was an FTIR peak appearing at 695 cm -1 , indicating the presence of quartz (SiO 2 ) 56 . The quartz phase was detected in the fresh MnTiO 3 -NW/SG and was clearly visible in all used catalysts, which is in good agreement with the XRD results in Fig. 7. This suggested that the α-cristobalite phase slowly transformed into the quartz phase. The FTIR peak at 524 cm −1 was assigned to the bending vibration of O-Mn-O 57 and it was detected in each catalyst. The intense peak at 590 cm −1 was the vibration of the O-Ti-O bond 58 , which was also perceived in each catalyst.
The plausible mechanism of this reaction over MnTiO 3 -NW/SG is illustrated in Scheme 2. The most possible active site for the CH 4 activation is the lattice oxygens of WO 4 2,30 . During the CH 4 activation to CH 3 radical by the lattice oxygens, W 6+ is reduced to W 5+ 30 . The CH 3 radical is then coupled with another CH 3 radical to yield C 2 H 6 in the gas phase, which can further generate C 2 H 4 and other hydrocarbons via the C-H activation and dehydrogenation over the catalyst surface. However, the oxidation of CH 4 can occur at the same temperature www.nature.com/scientificreports/ (600-1,000 ºC) to produce CO and CO 2 . A sketch of the mechanisms and series of OCM reaction, CH 4 oxidation, and dehydrogenation are presented in Supplementary Information equations (S1)-(S14) 19 . At the same time, due to the facile mobility of the lattice oxygen of Mn-O species 27 , the lattice oxygen from MnTiO 3 is transferred to the WO 4 2species. As a result, Mn 2+ is oxidized to Mn 3+ . After that, the O 2 molecule from the gas phase is adsorbed onto the surface of MnTiO 3 and the OH radical is released from the WO 4 -2 surface. An electron exchange from W 5+ to Mn 3+ simultaneously occurs, which regenerates W 6+ and Mn 2+ , and is ready for the new cycle of the reaction.
A survey of various catalysts reported in the literature for the OCM reaction is presented in Fig. 10, with the details of each catalyst described in Supplementary Table S17. The activity of OCM catalysts should be over 30% CH 4 conversion and 80% C 2+ selectivity 7 to have the potential for commercial exploitation (indicated by the gray area in Fig. 10). The catalysts studied herein are outside the required area; specifically, as their C 2+ selectivity is below 80%. Several reported catalysts also had a CH 4 conversion above 30% with a C 2+ selectivity below 80%, however, all of them were used at a temperature above 750 °C. Thus, in light of the lower temperature (700 °C), our present catalysts exhibited good performance. Nevertheless, further improvement is necessary, especially  www.nature.com/scientificreports/ increasing C 2+ selectivity above 80% while maintaining the CH 4 conversion. Some Mn-Na-W/Si and X-Na-W/ Si catalysts have a C 2+ selectivity of around 80% at a reaction temperature of 725-775 °C, but they have low CH 4 conversions (4.4-20.2%) [15][16][17]31 . No catalyst based on Na, W, and/or Mn can provide a C 2+ selectivity above 81%, implying that improving the C 2+ selectivity while maintaining the CH 4 conversion above 30% is very challenging. Remarkably, one report in 1998 showed that an Mn-Na-W/Si catalyst performed well in the OCM reaction, producing C 2+ selectivity of 80% and CH 4 conversion of 33% (for a yield of 26.4%) at a reaction temperature of 850°C 20 . Nevertheless, thereafter such high performance of the same catalyst was not reported. The addition of additives to Mn-Na-W/Si (as X-Mn-Na-W/Si) can improve the C 2+ selectivity to about 62-75% with a CH 4 conversion above 30%. When X = NaCl, the highest C 2+ yield reported was 34.6% (62.9% C 2+ selectivity and 55% CH 4 conversion) 17 , but the catalysts were not stable for long periods due to a loss of chloride. The performance of our catalysts containing Mn, Na, and W components upon using SiO 2 as catalyst support has been improved, identifying several key factors, but a viable catalyst for industrial use is still at large.

Conclusion
Different loadings of nanocomposite MnTiO 3 -NW or MnO x -TiO 2 -NW supported on silica (CS and SG) were successfully prepared and tested in the OCM reaction. The highest C 2+ yields were obtained for 10MnTiO 3 -NW/ SG (21.6%), followed by 20Mn-Ti-NW/SG (21.5%), and 5MnTiO 3 -NW/CS (20.6%). These catalysts produced high levels of dehydrogenation, generating olefins/paraffin ratios of 2.1-2.3. Catalysts characterization by XRD detected MnTiO 3 , MnO x , TiO 2 , and α-cristobalite phases, while using XPS identified oxygen species of M-O (M = Na, W, Mn, and Ti) that were strongly related to high CH 4 conversion and high C 2+ yield. Considering the catalytic activity between MnTiO 3 -NW and Mn-Ti-NW on the same support (i.e., SG), a small activity difference was observed between these two when the metal loading at 5wt%, in which the activity of 5MnTiO 3 -NW/ SG was lower than that of 5Mn-Ti-NW/SG, while they perceived no substantial difference in the activity at the higher loadings. Comparing the type SiO 2 support, the activity results presented that the catalysts with porous SG support were more stable than those with non-porous CS as support. The gradual catalyst deactivation observed during the stability test, especially for the catalyst with CS, was mainly due to the destruction of crystalline Na 2 WO 4 . Therefore, further improvement of catalytic performance seems to require an alternative active component that is retained under harsh operating conditions.

Methods
Catalysts preparation. Preparation of MnTiO 3 nanocatalysts. The MnTiO 3 catalyst was prepared using the stearic acid gel method 70  Catalyst activity testing. The catalytic activity of each prepared catalyst in the OCM reaction was evaluated in a plug flow reactor. A catalyst (50 mg) was packed in a borosilicate glass tube with an inner diameter of 5 mm and sandwiched between layers of quartz wool. The feed gases were methane (CH 4 , 99.999% HP, Labgaz), oxygen (O 2 , 99.999% HP, Linde), and nitrogen (N 2 , 99.999% UHP, Labgaz). The feed gas ratio of CH 4 :O 2 :N 2 was 3:1:4 at a total feed flow rate of 50 mL min -1 (GHSV = 30,558 h -1 ), which was fed into the plug flow reactor (Kejia furnace KJ-TI200). All flow rates were controlled using mass flow controllers (Aalborg GFC17) and doublechecked using a bubble flow meter. The operating conditions were atmospheric pressure and a reactor temperature of 700 °C. The catalyst activity was evaluated 1 h after the system had reached the established conditions. The quantification of the gas products was carried out by gas chromatography (SHIMADZU, GC-14A) using Unibead C column connected with a thermal conductivity detector (TCD) for determining CO, CO 2 , and CH 4 and Porapak Q column connected with a flame ionization detector (FID) for determining C 2 H 4 , C 2 H 6 , C 3 H 6 , C 3 H 8 , C 4 H 8 , and C 4 H 10 . A standard calibration curve for each product was established using five calibration points with a coefficient of determination (R 2 ) > 0.995. The activity of each catalyst is presented in terms of %CH 4 conversion, %C 2+ selectivity, %CO x selectivity, %C 2+ yield, olefins/paraffin ratio (mol/mol), and C 2+ formation rate (r C2+ ), which were calculated using Eqs. (1)- (6), respectively.
where n is the number of moles. The reported data is an average obtained from at least three separate catalytic tests. An example of carbon balance checks is shown in Supplementary Information Table S19.
Catalyst characterization. The morphology and metal dispersion of the catalysts were analyzed by field emission scanning electron microscopy with energy dispersive X-Ray spectroscopy (FE-SEM/EDS, FE-SEM: JEOL JSM7600F). Before the measurements, each catalyst was sputter-coated with platinum to increase the contrast for imaging. The structural properties of each catalyst were examined by Fourier-transform infrared spectroscopy (FTIR; PerkinElmer Spectrum 400 FT-IR/FT-FIR). For each spectrum, 32 scans were collected over a spectral range of 400-4,000 cm -1 at a resolution of 4 cm -1 .
The crystal structure of each catalyst was analyzed by powder X-ray diffractometry (XRD; Bruker D8 Advance), using Cu-Kα radiation at 45 kV and 40 mA with a step size of 0.02° and a step time of 0.5 s.
The surface area, pore size, and pore volume of each catalyst were measured by an N 2 -adsorption analyzer (3Flex Physisorption Micromeritics), following the Brunauer-Emmett-Teller (BET) method being conducted at −196 °C.
X-ray photoelectron spectroscopy (XPS; Kratos Axis Ultra DLD) was used to characterize the elements in each catalyst, namely sodium (Na 1 s), tungsten (W 4f.), manganese (Mn 2p), titanium (Ti 2p), silicon (Si 2p), and oxygen (O 1 s). The binding energy of C 1 s (285.0 eV) was used as a standard for all other binding energies.